Mark `preact` as externals for server bundle (#15110)

Co-authored-by: NJason Miller <developit@users.noreply.github.com>

examples/using-preact/next.config.js

@@ -14,6 +14,13 @@ module.exports = withPrefresh({
       }
     }
 
+    if (isServer) {
+      // mark `preact` stuffs as external for server bundle to prevent duplicate copies of preact
+      config.externals.push(
+        /^(preact|preact-render-to-string|preact-context-provider)([\\/]|$)/
+      )
+    }
+
     // Install webpack aliases:
     const aliases = config.resolve.alias || (config.resolve.alias = {})
     aliases.react = aliases['react-dom'] = 'preact/compat'